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If you have been growing corn and soybean or advising growers for several decades, it’s possible to remember how the ease of controlling weeds has switched back and forth between the two crops. There have been periods when control is easier in corn than soybeans (early days of atrazine) and then those when the reverse has been true (early days of Roundup Ready soybeans). The development of glyphosate resistance issues has resulted in a trend where currently several weeds are more effectively and/or less expensively controlled in corn than in soybeans. Or as Dickens might have said if he was a weed scientist: “It was the best of times in corn, it was the worst of times in soybeans.”

This is certainly not true for all growers, since some still have great success in Roundup Ready soybeans. We do believe, however, that for several tougher weeds that have developed glyphosate resistance – giant and common ragweed, marestail and waterhemp – it’s essential to get effective control in corn to reduce the population that has to be managed in soybeans. A number of growers have commented during winter meetings that there was way too much giant ragweed in corn at harvest this year, and we made the same observations. Problems with weather and crop development can contribute to this, but other possible causes that are affected directly by grower decision-making include:

Failure to recognize that it is mostly not possible to adequately control giant ragweed with a total PRE herbicide program, unless the infestation is extremely low.

Failure to use enough residual herbicide in a PRE + POST approach.

Not using the most appropriate timing of POST herbicide applications.

This article addresses the second problem for giant ragweed and other weeds: How much residual herbicide is needed in PRE + POST systems?